1) Herpesviruses cause latent infections that persist for the lifetime of the host and these viruses have developed mechanisms to counteract host defenses so as to allow the virus to persist. Using sophisticated computer programs we are identi-fying herpesvirus genes that may have cellular homologs. These genes are being expressed in various systems to determine how the genes may interact with host cell proteins, including those of the immune system, to influence the course of infec- tion. Identification of these genes may help define new targets for antiviral therapy or new insights into modulating the immune system. 2) Epstein-Barr virus (EBV) encodes a type II membrane protein, BZLF2, that is homologous to members of the C-type lectin family. In collaboration with Immunex Corporation, we have constructed a soluble fusion protein containing the human IgG Fc domain linked to the extracellular domain of BZLF2. The extracellular domain of BZLF2 was found to bind to the human MHC class II HLA-DR beta chain. Site directed mutagenesis of the HLA-DR beta chain indicated that the beta1 domain was required for recognition of the extracellular domain of BZLF2. Since this domain participates in forming the peptide binding pocket, these results suggested that BZLF2 may interfere with class II directed antigen presentation. The fusion protein containing the extracellular domain of BZLF2 inhibited antigen presentation and generation of antigen-specific cytotoxic T lymphocytes in mixed lymphocyte cultures. Current studies are directed toward inactivating the BZLF2 gene in EBV to determine the role of the protein during growth of the virus both in cell culture and in an animal model. 3) Vari-cella-zoster virus (VZV) ORF13 encodes the viral thymidylate synthetase. This protein has 68% amino acid identity to the human thymidylate synthetase. We have constructed a mutant in the Oka vaccine strain of VZV that cannot express the viral thymidylate synthetase by inserting stop codons into the gene. We plan to inocu-late guinea pigs with the VZV mutant to determine whether viral thymidylate synthe-tase is important for infection with the virus or for the ability to spread to the central nervous system and maintain latent infection. 4) Additional herpesvirus homologs of known mammalian proteins have been identified. Promising candidates will be studied in appropriate systems in collaboration with M. Spriggs at Immunex Corporation.